A planar X-ray detector in which an active matrix is used is developed as a new-generation diagnostic X-ray detector. An X-ray photographic image or a real-time X-ray image is output as a digital signal by detecting an X-ray with which the X-ray detector is irradiated. In the X-ray detector, the X-ray is converted into visible light, that is, fluorescence by a scintillator layer, and the fluorescence is converted into a signal charge by a photoelectric conversion element such as an amorphous silicon (a-Si) photodiode and a CCD (Charge Coupled Device), thereby obtaining an image.
Generally, for example, cesium iodide (CsI):sodium (Na), cesium iodide (CsI):thallium (Tl), sodium iodide (NaI), and gadolinium oxysulfide (Gd2O2S) are used as a material for the scintillator layer. A groove is formed by dicing, or deposition is performed by an evaporation method such that a pillar structure is formed, which allows improvement of a resolution characteristic of the scintillator layer. As described above, various materials can be used for the scintillator, and different materials are used for different applications and necessary characteristics.
There is a method for forming a reflection layer on the scintillator layer in order to enhance use efficiency of the fluorescence from the scintillator layer to improve a sensitivity characteristic. That is, in the fluorescence emitted from the scintillator layer, the fluorescence travelling toward the opposite side to the photoelectric conversion element side is reflected by the reflection layer to increase the fluorescence reaching the photoelectric conversion element side.
For example, a method for depositing a metallic layer, such as a silver alloy and aluminum having a high fluorescent reflectance, on the scintillator layer and a method for applying and forming the light-scattering reflection layer containing a binder resin and a light-scattering material such as TiO2 are well known as the reflection layer producing method. A method in which a reflection plate having a metallic surface made of aluminum and the like is not formed on the scintillator layer, but brought into close contact with the scintillator layer to reflect the scintillator light is also put to practical use.
A moistureproof structure that protects the scintillator layer and the reflection layer or the reflection plate from an outside atmosphere to prevent characteristic degradation caused by the humidity becomes an important constituent in an effort to make the radiation detector a practical product. Particularly high moistureproof performance is required when the CsI:Tl film or the CsI:Na film, which is of a material largely degraded by the humidity, is used as the scintillator layer.
For example, a method in which a poly-paraxylene CVD film with which the scintillator layer is covered is used as a moistureproof layer (for example, see Japanese Patent No. 3077941 (pages 3-4 and FIGS. 1 and 2)) and a method in which a cover adheres onto an enclosure member enclosing the surroundings of the scintillator layer with an adhesive agent to seal the scintillator layer while the enclosure member adheres onto a substrate with the adhesive agent (for example, see Jpn. Pat. Appln. KOKAI Publication No. 5-242841 (pages 3-5 and FIG. 1)) are disclosed as the conventional moistureproof structure.
However, there are following problems in the conventional moistureproof structure.
For the method in which the poly-paraxylene CVD film is used as the moistureproof layer, frequently a moisture transmission barrier characteristic is inadequate in at least a practical film thickness range (for example, 20 μm). In order to confirm the moistureproof performance, a sample in which the CsI:Tl film (thickness of 600 μm) that is the scintillator layer and the poly-paraxylene CVD film (thickness of 20 μm) that is the moistureproof layer are used on a glass substrate is made, and result of investigation of changes in luminance and resolution in a high-temperature and high-humidity test will roughly be described below.
In the method for measuring the luminance and the resolution, the X-ray was irradiated from the scintillator layer side, and an X-ray image was observed from the glass substrate side with a CCD camera by focusing on an interface between the glass substrate and the scintillator layer. The luminance was luminance relative to an intensifying screen (HG-H2 Back, product of Fujifilm Corporation), the resolution was obtained by measuring CTF (Contrast Transfer Function) of 2 Lp/mm from a resolution chart image, and the both were used as indexes.
For the sample produced in the above-described way, in a 60° C.-90% RH high-temperature and high-humidity life test, the change in luminance is small while the resolution was largely degraded, and a CTF (2 Lp/mm) value was decreased to about 80% of the initial state in 24 hours. After morphology was observed with a SEM as a phenomenon analysis of the decrease in resolution, it was found that, while the CsI:Tl film had the highly independent pillar structure in the initial state, fusion was generated between the pillars in the sample in which the resolution was degraded by the high-temperature and high-humidity test. This is attributed to the fact that a light guide effect is reduced by the fusion between the pillars to lead to the decrease in resolution.
For the structure in which the cover adheres onto the enclosure member enclosing the surroundings of the scintillator layer with the adhesive agent to seal the scintillator layer while the enclosure member adheres onto the substrate with the adhesive agent, generally the enclosure member is made of a material such as metal which has rigidity, a crack or peeling of an adhesive portion is easily generated in a reliability test of a cold cycle or thermal shock by a thermal expansion coefficient difference between the substrate and the enclosure member and between the cover and the enclosure member, and the moistureproof performance is fatally reduced. Because the adhesion and sealing are performed above and below the enclosure member, the amount of moisture transmitted through the resin adhesive agent is clearly increased when compared with the case of one adhesive portion.
In view of the foregoing, an object of the present invention is to provide a radiation detector that has the excellent moistureproof performance and the high reliability for the temperature change of the cold cycle or thermal shock and a method for producing the same.